A site by a Biology tutor with lots of experience, there are many useful things here including video guides, past papers, topic guides etc. etc.
A revision notes site set up by an A level student. Not all topics here yet, those that are include cells and biological molecules.
Animation on the Krebs cycle followed by a quiz to check knowledge.
Blog consisting of revision notes for A level biology topics – AQA spec. Revision note style with diagrams.
Past papers and mark schemes for AQA are available here
Multiple choice quizzes on many biology topics – written for AP Biology (US course) but many topics are at a similar level to A level.